For several decades there has been an increasing usage of a variety of plastics, such as fiberglass reinforced plastics (also called FRP), as materials of choice for storing and/or transporting a wide variety of fluids, such as water, fuels, chemicals, compressed gases, waste products, brine, fumed products, and the like. The choice of plastic usually depends, at least in part, on the amount of chemical and/or solvent inertness required and also on the abrasiveness and temperature of the product being stored or transported. The containers can range from relatively small sizes, such as drums, to very large sizes, such as storage tanks, rail tank cars or shipping barges, or very elongated containers, such as pipes or ducts or other conduits.
At times the plastic, including reinforced or laminated plastic, comprises essentially the entire wall structure of the container, and sometimes a container is merely lined with plastic. In any event, whether the plastic comprises the entire container or only serves as a liner in a container made of some other material (usually metal or a composite material) the rate of wear, damage, or deterioration may require testing or monitoring, especially if a leakage of fluid stored or carried in the container might create an unacceptable detriment to flora and/or fauna, or constitute a violation of the law. Such problems can be appreciably avoided if excessive wear of the container wall or damage to the container wall is detected soon enough by monitoring electrical changes in the wall due to permeation or intrusion of the contained fluid into the container wall. It has been determined that virtually any fluid contained in a non-conductive, inert container, while often being a fluid having very poor electrical conductivity, can create a detectable change in the electrical properties of the non-conductive inert container by intrusion or permeation of the fluid into the non-conductive, inert container wall at points of imperfect construction, damage or degradation if means can be provided for monitoring the electrical properties using non-destructive testing. Fluids which are essentially electrically non-conductive, but which may intrude into the wall in which they are contained, also pose a problem for which a remedy is needed.
As disclosed in Chemical Engineering, page 464, Jul. 1991, a means has been developed for detecting changes in the electrical properties of a container wall comprising FRP which the gradual intrusion of a stored aggressive substance into the wall can be monitored. The means involves, as the monitoring device, a set of probes embedded at various depths in predetermined areas in the FRP wall to operate in circuitry with a "wet" probe which penetrates through the wall to be in contact with the fluid within the container. The embedded probes and the wet probe are each connected to an electrical receptacle mounted on the exterior of the wall with a means for connection to a microammeter. In a related brochure it is suggested that in areas of secondary joints, such as incoming pipes, a graphite fiber mat that completely encompasses the joint can be substituted for a probe. The above-described means and method, which monitors a relatively small area in which the embedded probes and their cooperating wet probe are located, may fail to give an early warning of a damaged or badly eroded container wall at a location which is not near enough to the probes to be effectively indicated by the microammeter.
The use of a carbon or graphite veil behind a non-conductive thermoplastic liner for a thermoset fiberglass structural tank has been used to allow spark testing of the thermoplastic liner to assure there are no micro holes or cracks in the liner that would allow a corrosive material to get behind the liner and into the thermoset fiberglass structural part of the tank. A spark generator is used for finding tiny holes or cracks that are too small to be seen by the naked eye; any such imperfections in the thermoplastic liner which reach the carbon or graphite veil will provide a path to "ground" which is evidenced by a visibly strong spark from the spark generator to the hole or crack. This type of test does not require that an electrical conductor wire be connected to the conductive layer and protrude to a location outside the wall of the tank for connection to an electrical measuring instrument. Also, this type of testing is used as a quality test for new vessels, where the spark generator is normally swept across the inner surface of the container before being placed in service. The spark generator can also be used when the container has been emptied for cleaning or inspection and the inner surface of the container re-tested. The spark generator is not used to test the inside surface of containers while the containers are filled with fluid.
It is an object of this invention to provide a means and method which can be used in monitoring the electrical properties across a liner, essentially of an entire container, or at least a relatively large portion of a large container, and not rely simply on many probes which monitor only a relatively small area per probe.
It is also an object of this invention to employ a "wet" probe which can be removed when no measurements are being made, or if mounted therein can be easily replaced if damaged, without being permanently mounted as projections through the inner surface of the container wall per se, whereby there can form a crevice around the probe which, itself, can promote degradation of the wall due to intrusion of the stored fluid into the crevice around the projecting probe, especially where changes in temperature coupled with different coefficients of expansion can cause leaks.
It has now been discovered that an electrically insulating material (i.e. a non-conductive material) used as a substantially inert material to contain a fluid that has at least some electrical conductivity, can have its fluid-containment ability monitored by measuring the electrical properties across the non-conductive material between the fluid in the container and an electrical conductor co-extensively in contact with and surrounding the electrically insulating, substantially inert material. Penetration or intrusion of the electrically conductive liquid into or through the liner, will result in a loss in the electrical insulation properties; the loss can be detected and/or monitored and the data gathered can be used to predict or determine when the inertness or integrity of the containment layer will no longer be capable of satisfactorily containing the liquid.
Ways and means have now been developed for detecting and/or monitoring damage, degradation or deterioration of any part of a containment wall of a fluid container (e.g., tank, barge, pipe, duct, pump, flume, etc.) wherein there is an electrically conductive fluid contained within an electrically-insulating material such as plastic, rubber, ceramic, refractory and the like. In the present invention, described more fully below, there is provided as a part of the containment wall, at least one electrically-conductive veil or layer which is separated from the contained fluid by the chemically-resistant electrically-insulating material, the said conductive veil or layer being firmly attached co-extensively to the electrically-insulating containment layer of the wall. The conductive veil or layer is provided with attached means for connecting at least one conductor wire to an instrument for measuring electrical properties, whereby intrusion of the fluid into the containment wall can be detected and/or monitored.
As used in this disclosure, the term "inert layer" means that the containment layer of the multi-layer wall is essentially chemically inert to, or is not readily dissolved or otherwise substantially attacked by, the fluid in the containment portion of the fluid container. The terms "non-conductive" and "conductive" refer to electrical properties unless stated otherwise, it being understood by practitioners of these relevant arts that if the conductivity is low enough (essentially nil for all practical purposes) then it may be non-conductive enough to function as an electrical insulator depending on the relative level of electrical power involved. The terms "conductor", "conductive wire" and "wire" are used in describing a means for electrically connecting portions of the containers to instruments or to other conductors to provide circuitry for measuring electrical properties. Generally a metal wire is used, though conductive ribbons, rods, webs, meshes, graphite tows or strips or other configurations are within the purview of the present inventive concept.
When using instruments to measure electrical properties, practitioners will be expected to employ the usual procedure of impressing a DC voltage through appropriate circuitry and "zeroing" the instrument to provide a reference point against which to compare subsequent measurements of the circuit to be tested. The voltage can be from less than 1 volt to about 2,500 volts or more, depending on which electrical property one wishes to measure. About 500 to about 1000 volts is generally suitable for measuring megohms in the present invention when testing containers which contain non-flammable fluids, though different voltages may be found to be better in measuring other electrical properties such as amperage, capacitance, power factor, dielectric constant, loss factor, or scanning for voids using spark generators using voltages up to about 50,000. When measuring containers that contain flammable fluids, it is recommended that very low voltages be used, especially voltages of less than 1 volt, in order to avoid creation of a spark where flammable vapors could be ignited.